Structural Dynamics of 1,2-Diiodoethane in Cyclohexane Probed by Picosecond X-ray Liquidography

We investigate the structural dynamics of iodine elimination reaction of 1,2-diiodoethane (C2H4I2) in cyclohexane by applying time-resolved X-ray liquidography (TRXL). The TRXL technique combines structural sensitivity of X-ray diffraction and 100 ps time resolution of X-ray pulses from synchrotron and allows direct probing of transient structure of reacting molecules. From the analysis of time-dependent X-ray solution scattering patterns using global fitting based on DFT calculation and MD simulation, we elucidate the kinetics and structure of transient intermediates resulting from photodissociation of C2H4I2. In particular, the effect of solvent on the reaction kinetics and pathways is examined by comparison with an earlier TRXL study on the same reaction in methanol. In cyclohexane, the C2H4I radical intermediate undergoes two branched reaction pathways, formation of C2H4I-I isomer and direct dissociation into C2H4 and I, while only isomer formation occurs in methanol. Also, the C2H4I-I isomer has a shorter lifetime in cyclohexane by an order of magnitude than in methanol. The difference in the reaction dynamics in the two solvents is accounted for by the difference in solvent polarity. In addition, we determine that the C2H4I radical has a bridged structure, not a classical structure, in cyclohexane.